Traditionally, the referee or referees of a sports match decides from visual observation whether or not the ball has passed the goal plane. However, this may be very difficult to determine correctly in situations where the ball is returned quickly and has only just passed, or not passed, the goal plane, and it is particularly difficult if the referee is positioned unsuitably with respect to the goal plane or is engaged in other activity of the match. Video camera may also be used to monitor the goal planes, but the spatial and temporal resolution of video cameras are often not sufficient to provide the necessary information in cases of doubt.
A number of electronic systems are known in the art for determining the position of a ball on a sports field by means of position systems, as disclosed in e.g. WO 01/66201, FR 2 753 633, FR 2 726 370, WO 99/34230, U.S. Pat. No. 4,675,816, U.S. Pat. No. 5,346,210 and WO 98/37932. These positioning systems may be used e.g. for determining if the ball has passed the border of the playing field and the positions of the players as well and provides many useful information to the referee. However, the determination of the passage of the goal plane is a very delicate matter, both because it may be decisive for the outcome of the sports match and because the distances are small and the velocity of the object often very high, so that a position determining system to provide a reliable determination of whether the object has passed the goal plane must be very precise in the determination of the position and at the same time have a very high update rate of the position determination. The object may e.g. move with 72 km/h or 20 m/s, which means that an update rate of 1/100 s will add an uncertainty of 20 cm to the determined position, which is unacceptable with respect to determination of a goal in a sports match.
WO 00/47291 and U.S. Pat. No. 4,375,289 discloses devices for detecting the position of a moving object relative to a plane, wherein an energizing coil generates an electromagnetic field, which is disturbed by the moving object. The disturbance is detected by one or two coils from which it is decided whether and when the object passed the plane encircled by the energizing coil as well as the detection coils. These devices requires the coils to encircle the full goal plane and are very sensitive to any deviation from the precise and correct positioning of the coils as no calibration for such deviations is available, and the detection may also be severely disturbed by other object in or near the goal plane, such as a goal keeper or other players. Furthermore, players within the vicinity of the goal plane will be exposed to the generated electromagnetic field, a fact that may raise health related concerns.
Position systems with a sufficiently precise determination of the position of a sports object and a sufficiently high update rate to provide reliable indications of the crossing of a goal plane, are very expensive to install and maintain. It is therefore desirable to provide an alternative system with a sufficient spatial as well as temporal resolution to provide reliable indications.
U.S. Pat. No. 5,976,038 discloses an apparatus for providing an output indication when a playing object crosses the play determinative line. The apparatus comprises a directional receiving antenna, such as a disk-reflector antenna and in particular a cassegrain antenna provided with dual, horizontally adjacent feeds, which are combined to provide sum and difference signals. The antenna is arranged outside the playing field and is directed along the play determinative line. In order to provide a sufficiently high spatial resolution due to the distance between the antenna and the playing object, the reflector of the antenna must have considerable dimensions. A reflector of 30 inch width, 76 cm, will provide a detection zone of 4 inch width, 10 cm, which together with other uncertainties of the system is acceptable for use with American football as the patent is directed at, but is unacceptable for many other sports games and a much larger reflector would be required.
It is therefore desirable to provide a technically more simple system for determining the crossing of a goal plane with a sufficient spatial as well as temporal resolution to provide reliable indications.